Umami is the fifth taste. For a long time people in the West said there were four — sweet, sour, salty, bitter — and then a Japanese chemist, Kikunae Ikeda, argued in 1908 that there is another one, a deep savoury taste that none of those four describes. That taste is umami. You know it already: the taste in a good bowl of beef noodle soup, in mushrooms, in soy sauce, in aged cheese, in tomatoes that have been cooked a long time, in dried scallops and kombu seaweed. It makes food taste full and round, and it makes your mouth water and want more. The word works two ways in English. As a noun: "this soup has a lot of umami". As an adjective before a noun: "an umami flavour", "umami-rich ingredients". Both are common. English needed to borrow this word because it had no good one of its own — "savoury" is the closest, but savoury really just means "not sweet", which is a much wider idea. Note that it is a fairly new word in English. It arrived properly in the 1980s and 90s with food writing, and for a while it sounded fashionable, the kind of thing a chef on television says. It is now normal in cooking and food articles, but you would not usually say it at a family dinner table; most English speakers would just say "wow, this is really tasty". Scientifically, the taste comes from glutamate and a couple of related substances — which is exactly what MSG (味精) is made of.